The conventional communication method will be explained first. The CSMA, which is one of the random access technologies, is adopted, for example, in a communication system performing wireless packet communication and the like. In the CSMA, a plurality of radio terminals constituting the system performs carrier-senses for a radio channel prior to wireless packet transmission. When it is recognized that the channel is being used (channel busy), transmission of the wireless packet is suspended, and thereafter, when it is recognized that the channel is not being used (channel idle), the wireless packet is transmitted.
In this communication system, however, there are cases that a transmission signal from another radio terminal cannot be directly received, such as a case that the radio terminals are located with such a distance therebetween that radio waves do not reach, or a case that there is an obstacle, which blocks radio waves, between the radio terminals. The radio terminal whose presence cannot be recognized, in spite of constituting the same communication system, is referred to as a “hidden terminal”. Since the carrier sense does not work effectively between the radio terminals corresponding to the hidden terminals, there is a case that one terminal starts transmission of a wireless packet, while the other is transmitting a wireless packet, and in such a case, for example, a collision of wireless packets occurs in a radio base station located at an intermediate position between the radio terminals, thereby disabling normal communication.
In Wireless LAN Standard IEEE802.11, the problem of hidden terminals is remedied by request-to-send (RTS)/clear-to-send (CTS) of a distributed coordination function (DCF) using the carrier sense multiple access/collision avoidance (CSMA/CA).
A communication method between a base station (AP) and a radio terminal (STA) in a wireless LAN system adopting the CSMA will be explained below. It is assumed here that communication is performed in such a state that STA (1) belongs to AP (1), and STA (2) belongs to AP (2). The STA (2) is in a communication range of the STA (1), and the STA (1) is in an interference range of the STA (2). A packet format of the RTS and the CTS used in the system includes a packet type field for discriminating the packets of RTS and CTS from each other, a destination address field, a sender address field (not in the CTS), a channel use period field by a transmitted wireless packet, and an error-checking-code field for checking a bit error in the packet.
Firstly, the AP (1) transmits an RTS frame, which is a control frame, to the STA (1). The STA (1) transmits a CTS frame to the AP (1). The respective frames include virtual carrier sense information referred to as a net allocation vector (NAV), indicating, for example, the channel use period in the communication with a radio terminal corresponding to the destination address. Therefore, transmission from a radio terminal other than the destination address is suspended until the time (period) specified in the NAV. That is, here, the STA (2) is in the transmission-suspended state.
The AP (1) having received the CTS frame then transmits a DATA frame to the STA (1). The STA (1) having received the DATA frame sends back an ACK frame to the AP (1).
On the other hand, the STA (2) having received the CTS frame from the STA (1) cannot send back the CTS frame even when it has received the RTS frame from the AP (2), since it is in the transmission-suspended state by the NAV. Since the CTS frame is not sent back from the STA (2), the AP (2) retransmits the RTS frame until the transmission-suspended state of the STA (2) is cancelled. When the number of retransmission reaches a preset upper limit, the AP (2) can cancel the frame.
When the AP (2) retransmits the RTS frame to the STA (2) in the state that the channel use period is expired and the transmission-suspended state is cancelled, the STA (2) transmits the CTS frame to the AP (2). The AP (2) having received the CTS frame transmits a DATA frame to the STA (2), and the STA (2) having received the DATA frame sends back the ACK frame to the AP (2).
Thus, in the conventional wireless LAN system, a plurality of radio terminals is connected to a plurality of base stations operating in the same frequency, and when there is a hidden terminal due to the interference between the radio terminals connected to different base stations, a collision of packets can be avoided by the RTS/CTS.
In the conventional communication method, however, for example, the STA (2) is turned to the transmission-suspended state due to the NAV of the STA (1) connected to the other AP (1) operating in the same frequency. Therefore, in the transmission-suspended state, the STA (2) cannot transmit the ACK frame, even when it can receive the DATA frame from the AP (1), thereby causing a drop in the throughput considerably.
As a method of avoiding a drop in the throughput, for example, there are a method of controlling a downlink and an uplink timewise by synchronizing a plurality of base stations, and a method of adjusting the distance between the base stations. However, with these methods, there is a problem in that a plurality of companies or individuals cannot install the base station at random.
The present invention has been achieved in order to solve the above problems. It is an object of the present invention to provide a communication method capable of avoiding a drop in the throughput, without performing synchronous control between the base stations and distance adjustment between the base stations.